1. Field of the Invention
The present invention relates to a dividing method for a wafer by which a semiconductor wafer is divided along a street.
2. Description of the Related Art
A semiconductor wafer is partitioned into a large number of rectangular regions with streets (planned dividing lines) arrayed in a lattice pattern and are divided into individual chips along the streets. Conventionally, as a dividing method for a wafer, a method of dividing a wafer into individual chips by combination of SD (Stealth Dicing) and expansion (refer to, for example, Japanese Patent Laid-Open No. 2005-252126 and Japanese Patent Laid-Open No. 2009-111147). In the SD, a laser beam is irradiated along a street to form a modified region, which becomes a starting point of division, in a wafer. In the expansion, a tape pasted to a wafer is expanded so that external force is applied to a modified region to divide the wafer along a street.
Meanwhile, in the dividing method for a wafer disclosed in Japanese Patent Laid-Open No. 2005-252126 and Japanese Patent Laid-Open No. 2009-111147, since tensile force is applied to a modified region in which a strength decreases so that the wafer is broken at the modified region, if a crack appears on a side face (dividing face) of a chip, then very small broken pieces scatter. Further, if a modified region having reduced strength remains on a side face of a chip after broken, then part of the modified region sometimes comes off and further scatters as a broken piece to ambient. Since such a crack or very small broken piece has a bad influence on a performance of a chip, etching gas is used to remove a modified region, a crack and a broken piece to smoothen a side face of a chip.